Implantable cardiac stimulation devices, particularly pacemakers and implantable cardioverter defibrillators (ICDs), are usually configured to be used in conjunction with an external programmer which enables a physician to program the operation of an implanted device to, for example, control the specific parameters by which the pacemaker detects arrhythmia conditions and responds thereto. For instance, the physician may specify the sensitivity with which the pacemaker or ICD senses electrical signals within the heart and also specify the amount of electrical energy to be employed in pacing pulses or defibrillation shocks. Another common control parameter is the atrioventricular (AV) delay, which for dual chamber devices specifies the time delay between a paced or sensed atrial event and a ventricular output pulse. Additionally, the external programmer may be configured to receive and display a wide variety of diagnostic information detected by the implantable device, such as intracardiac electrogram (IEGM) signals sensed by the device.
Insofar as programming is concerned, some control parameters specify operational algorithms to be performed by the implanted device including, for example, the mode of the device, such as whether the device is to operate in a dual-chambered mode or a single-chambered mode, the type of response to be performed if a pacemaker mediated tachycardia (PMT) or a pre-ventricular contraction (PVC) is detected, and whether any rate responsive sensors of the device are to be turned on or off (such as minute ventilation sensors). Other control parameters specify particular values to be use such as the pacing base rate, maximum tracking rate, minimum tracking rate, sensor rate, sensor slope, sensor threshold or AV delay of the implanted device. Together, the various control parameters permit the operation of the cardiac stimulation device to be tailored to the needs of a particular patient to provide optimal therapy while minimizing the risk of any unnecessary therapy. State of the art implantable cardiac stimulation devices may have dozens or hundreds of programmable parameters that can be individually programmed using the external programmer.
For many patients, particularly those with congestive heart failure (CHF), it is desirable to identify a set of control parameters that will yield optimal cardiac performance (also referred to as hemodynamic performance). Cardiac performance is a measure of the overall effectiveness of the cardiac system of a patient and is typically represented in terms of, one or more of, stroke volume, cardiac output, end-diastolic volume, end-systolic volume, ejection fraction, cardiac output index, pulmonary capillary wedge pressure, central venous pressure. Stroke volume is the amount of blood ejected from the left ventricle during systole. Cardiac output is the volume of blood pumped by the left ventricle per minute (or stroke volume times the heart rate). End-diastolic volume is the volume of blood in the chamber at the end of the diastolic phase, when the chamber is at its fullest. End-systolic volume is the volume of blood in the chamber at the end of the systolic phase, when the chamber contains the least volume. Ejection fraction is percentage of the end-diastolic volume ejected by the ventricle per beat. Cardiac index is the volume of blood ejected per minute normalized to the body surface area of the patient. Other factors representative of cardiac performance include the contractility of the left ventricle, the maximum rate of change of pressure with time (i.e. max dP/dt) or the maximum flow through mitral valve. Pulmonary capillary wedge pressure reflects filling pressure of the left ventricle. Central venous pressure reflects the fluid status of the patient. Cardiac performance can be assessed in a variety of ways, such as by measuring stroke volume using Doppler echocardiography, nuclear imaging, or thermodilution; or by measuring dP/dt using a pressure catheter. Pulmonary capillary wedge pressure can be measured using a pressure catheter in the pulmonary artery. Central venous pressure can be measured using a pressure catheter in the right ventricle or left atrium.
In view of the importance of maintaining optimal cardiac performance, especially for patients with compromised cardiac function, it would be desirable to provide improved techniques for use with pacemakers or ICDs for identifying pacing control parameters that optimize cardiac performance, particularly to reduce the degree of heart failure. It is to this end that aspects of the invention are generally directed.
For example, it is desirable to identify the AV delay value providing the best cardiac performance. In normal patients, the electrical conduction through the AV node is intact, and the body automatically adjusts the delay via the circulating hormones and the autonomic nervous system according to its physiologic state. It is well known, for example, that in normal patients the AV delay shortens with increasing heart rate. For patients with abnormal AV node conduction or complete heart block, a pacemaker can control the AV delay by delivering a ventricular pacing pulse at a software-controlled delay after an atrial pace or atrial sensed event. Since the optimum AV delay varies from person to person, this parameter must be optimized on an individual basis. Conventionally, the physician attempts to program the AV delay (or other parameters) for a given patient by using an external programmer to control the device implanted within the patient to cycle through a set of different AV delay values. For each value, the implanted device paces the heart of the patient for at least a few minutes to permit hemodynamic equilibration, then the physician records a measure of the resulting cardiac performance. The AV delay value that yields the best cardiac performance is then selected and programmed into the device.
Test phases for a conventional optimization procedure wherein each phase lasts four minutes are illustrated within FIG. 1. FIG. 2 illustrates a simulated cardiac performance curve 2 as a function of AV delay. Each triangle 4 in FIG. 2 represents the cardiac performance achieved at a particular test value of the AV delay. Circle 6 identifies the optimal AV delay value (normalized to 1). The traditional view has been that two to five minutes at each parameter setting is necessary to allow the cardiovascular system of the patient to equilibrate to provide reliable values of the resulting cardiac performance. Unfortunately, this can be time consuming—particularly if there are several different parameters to be optimized. In the example of FIG. 2, with eight different AV delay values tested using four minutes per value, thirty-two minutes is required just to obtain data to optimize the AV delay. Thus, the approach is not amenable to anything other than infrequent use in implantable devices.
One proposed technique for reducing the time required to optimize pacing parameters is set forth in U.S. Pat. No. 5,487,752 to Salo, et al. Briefly, Salo, et al. suggest that parameter optimization can be achieved by alternating between baseline pacing parameters and test pacing parameters, with the baseline parameters used for periods of about twenty heart beats and test parameters used for periods of about five beats. The test phases of this technique are illustrated within FIG. 3. According to Salo, et al., by limiting test periods to only five beats at a time and by alternating between test periods and considerably longer baseline periods, the cardiovascular system remains substantially in a baseline hemodynamic state and hemodynamic feedback systems are not significantly influenced, permitting the effects of the test parameters to be reliably determined despite the short test periods. As a result, Salo et al. assert that pacing parameters can be more quickly optimized than with predecessor techniques that pace for several minutes at each of the various test settings. Salo, et al. state that a ratio of 4:1 of baseline pacing to test pacing is desired to ensure that that the cardiovascular system remains substantially in the baseline hemodynamic state so that relatively short test periods can be accommodated. Assuming a heart rate of 60 beats per minute (bpm), the technique of Salo et al. would appear to still require about nearly 3½ minutes to complete an AV delay test using eight different AV delay values.
A related technique is set forth in U.S. Pat. No. 5,800,471 to Baumann et al. wherein a 3:1 ratio of baseline pacing to test parameter pacing is employed. Again the extended baseline period appears to be provided to allow for the hemodynamic system to return to a state of equilibrium between brief test periods.
Although the techniques of Salo, et al. and Baumann et al. appear to provide improvement over conventional techniques, it would be desirable to achieve an even greater reduction in parameter optimization times and it is to this end that further aspects of the invention are drawn. In particular, it is desirable to provide a technique for parameter optimization that would be sufficiently fast to allow for very frequent or semi-continuous parameter optimization so as to allow control parameters to be updated more or less continuously based upon the current needs of the patient. For example, the parameters could be updated to reflect changes in patient posture to provide a different set of control parameters optimized for use while the patient is standing as opposed to sitting or optimized for use while the patient is exercising as opposed to resting. As can be appreciated, with sufficiently fast parameter optimization, control parameters could be updated to provide optimal pacing for the patient at all times. It is to this end that still other aspects of the invention are drawn.
AV delay is just one example of a pacing parameter that is preferably optimized to achieve the best possible cardiac performance. Another is interventricular delay, which specifies the time delay between pacing pulses delivered to the right and left ventricles. In this regard, one factor associated with heart failure is asynchronous activation of the ventricles such that the mechanical contraction is not coordinated effectively thus compromising cardiac performance. As a result, the pumping ability of the heart is diminished and the patient experiences shortness of breath, fatigue, swelling, and other debilitating symptoms. The weakened heart is also susceptible to potentially lethal ventricular tachyarrhythmias. A decrease in cardiac performance can result from a progression of heart failure. In many cases, the interventricular delay can be adjusted to help improve cardiac performance and reduce the degree of heart failure, effectively reducing symptoms and improving the quality of life.
One particularly promising technique for reducing the risk of heart failure is “cardiac resynchronization therapy”, which seeks to normalize asynchronous cardiac electrical activation and the resultant asynchronous contractions by delivering synchronized pacing stimulus to both ventricles using pacemakers or ICDs equipped with biventricular pacing capability. The stimulus is synchronized so as to help to improve overall cardiac performance. This may have the additional beneficial effect of reducing the susceptibility to life-threatening tachyarrhythmias. In any case, biventricular pacing control parameters, such as interventricular delay, need to be adjusted so as to synchronize the ventricles and to optimize patient cardiac performance. As with the optimization of AV delay values, the optimization of interventricular delay values or other cardiac resynchronization therapy controls values can be time consuming.
Accordingly, it is also desirable to provide rapid optimization techniques for a wide variety of control parameters, particularly including parameters related to cardiac resynchronization therapy and the invention is directed to this end as well.